Conventionally, a vacuum pumping apparatus making use of a vacuum pump has been used to evacuate a predetermined container. The conventionally used vacuum pumping apparatus 1 disclosed in Japanese Laid-Open Patent Publication No. 8-42487 is shown in FIG. 12. The vacuum pumping apparatus 1 comprises a vacuum pump (pump) 4 for evacuating a vacuum container 2 by discharging the air through a valve 3, a motor 5 for driving the vacuum pump 4, an inverter 7 for adjusting power transmission from a power source 6 to the motor 5, and a controller 8 for controlling the inverter 7. The above-described apparatus 1 is equipped with a mode for operating the pump at a low speed of revolution, in order to prevent condensed or sublimated gas from adhering onto the inner wall of the pump.
However, if the motor 5 becomes overloaded for some reason, the above-described pumping apparatus 1 does not take any effective countermeasure. Therefore, if the motor 5 becomes overloaded, rotary drive of the motor 5 has to be discontinued to protect the motor 5. In consequence, the above-described pumping apparatus 1 can not keep the vacuum condition any longer. Accordingly, products being processed in the vacuum container become defective, contributing to various problems including increased costs.